Generally, silver halide grains are formed by allowing an aqueous silver salt solution and an aqueous halide solution to react in a reactor in the presence of an aqueous colloid solution. Two methods are known: (1) the single-jet method (hereinafter abbreviated as the SJ method) in which an aqueous silver salt solution is added with stirring to the mixture of a protective colloid (e.g. gelatin) and an aqueous halide solution for a prescribed period of time; and (2) the double-jet method (hereinafter abbreviated as the DJ method) in which an aqueous halide solution and an aqueous silver salt solution are added to an aqueous protective colloid solution for a prescribed period of time Advantages of the DJ method over the SJ method are that the silver halide grains with a narrower size distribution can be obtained and that the halide compositions of grains can be changed freely during their growth.
It is known that the growth rate of silver halide grains greatly depends on such factors as the silver (or halide) ion concentration of a reaction liquid, the concentration of a solvent for a silver halide, the distance between grains and the size of grains. If silver (or halide) ions are present in a reactor unhomogeneously, the grain growth rate may vary from grain to grain, resulting in the formation of silver halide grains lacking uniformity. To obtain silver halide grains being uniform in size, crystal structure, halide composition and other factors, it is important to allow an aqueous silver salt solution and an aqueous halide solution to react rapidly in an aqueous colloid solution (a parent liquid where formation, growth, and adjustment of emulsion grains will be performed) by mixing them uniformly. According to conventional methods, an aqueous halide solution and an aqueous silver salt solution are added to the surface of a parent liquid that has been put in a tank. In these methods, the concentrations of silver and halide ions tend to be higher in the vicinity of the inlets for these solutions than other places of the tank, and therefore, it is almost impossible to prepare silver halide grains being uniform in properties. To solve this problem, U.S. Pat. Nos. 3,415,650, 3,692,283 and British Patent No. 1,323,464 each propose a method which comprises supplying an aqueous halide solution and an aqueous silver salt solution to an oval, rotating mixer provided in a parent liquid tank through a pipe from its upper and lower open ends, allowing them to react rapidly by mixing them vigorously, thereby forming silver halide grains, and discharging the formed silver halide grains to the parent liquid tank by using a centrifugal force generated by the rotation of the mixer.
Japanese Patent Examined Publication No. 10545/1980 discloses a method comprising immersing a rectifying cylinder in a parent liquid tank, supplying reaction liquids separately to the cylinder from the bottom thereof, mixing the liquid vigorously with a turbine blade provided at the lower part of the cylinder, thus forming silver halide grains, and discharging the formed silver halide grains to the parent liquid tank from the opening provided at the upper part of the cylinder.
Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 92523/1982 discloses a method comprising immersing a mixer in a parent liquid tank, supplying an aqueous halide solution and an aqueous silver salt solution separately to the mixer, diluting these solutions with the parent liquid, and mixing them vigorously with shearing, thus forming silver halide grains.
By these conventional methods, though silver and halide ions can be distributed uniformly in a parent liquid tank, uniform distribution of these ions in a mixer cannot be realized. In a mixer, silver and halide ions tend to gather around the nozzles from which an aqueous silver salt solution and an aqueous halide solution are injected, the bottom of the mixer, or the stirring blade. Silver halide grains which are supplied with protective colloid to a mixer in which silver and halide ions are present unhomogeneously cannot grow at the same rate. Such difference in growth rate inevitably results in the formation of silver halide grains which differ from each other in size, halogen composition and other properties.
To overcome the drawback accompanying the above methods,proposed was a method that comprises supplying an aqueous silver salt solution and an aqueous halide solution to a mixer provided outside a parent liquid tank, mixing them vigorously to form silver halide grains, and supplying the formed grains to the parent liquid tank. For instance,Japanese Patent O.P.I. Publication No. 37414/1978 and Japanese Patent Examined Publication No. 21045/1973 each disclose a method which comprises circulating a parent liquid, supplying an aqueous silver salt solution, an aqueous halide solution and the parent liquid in a mixer provided in the middle of the parent liquid circulating line, mixing them vigorously in the mixer, while maintaining the ununiformity of the reaction system. Similar methods are disclosed in U.S. Pat. No. 3,897,935 and Japanese Patent O.P.I.Publication No. 47397/1978. In any of the above methods, the flow rate of the circulating parent liquid and the stirring efficiency of the mixer can be changed separately, thus enabling silver halide grains to be grown with silver and halide ions being distributed uniformly. These methods,however, are still defective in that silver halide grains supplied from the parent liquid tank to the mixer together with the parent liquid are caused to grow rapidly in the vicinity of the inlets for the aqueous silver salt solution and the aqueous halide solution. It means that, even by these methods, it is impossible to prevent perfectly the concentration of silver or halide ions from getting higher in the vicinity of the reaction liquid inlets or the stirring blade of the mixer.
To attain uniform distribution of silver and halide ions in a parent liquid, Japanese Patent O.P.I. Publication Nos.65925/1973, 88017/1976, 153428/1977, 99751/1987, J. Col. Int.Sci. 63 (1978) No. 1, page 16 and P.S.E. 28 (1984) No. 4,page 137 each describe a method in which silver halide grains that have been prepared separately are added to silver halide grains to be grown, allowing them to undergo the Ostwald' ripening. This method, however, has such a problem that, since the sizes of the silver halide grains to be added aren't small enough as compared with those of the grains to be grown, a lot of time is required for the completion of the Ostwald's ripening. By this method, growing of silver halide grains takes a prolonged period of time, resulting in high production cost and poor productivity.
As methods for forming silver halide fine grains, Japanese Patent O.P.I. Publication Nos. 183417/1989,183645/1989, Wo Nos. 89-06830 and 89-06831 each disclose a method in which silver halide fine grains are formed in a mixer provided outside a parent liquid tank, and immediately after their formation, the fine grains are supplied to the parent liquid tank. By this method, it is possible to obtain silver halide fine grains using relatively thin solutions of a silver salt and a halide. However, when use are made of thick solutions of a silver salt and a halide, since these solutions are allowed to collide with each other in a mixer by stirring, there may arise such a problem that even a small change in the flow rates of these solutions may be attended by a considerable change in pAg, i.e., silver ion concentration. Another problem accompanying this method is that, at some pAg values, silver halide fine grains with reduced silver nuclei tend to be formed in the mixer. These fine grains., when supplied to the parent liquid tank, arere-dissolved into silver and halide ions, and incorporated into growing grains together with their reduced silver nuclei. As a result, some of the emulsion grains formed by this method contain reduced silver nuclei, which may cause a photographic image obtained by using these emulsions to be fogged.
Still another defect of the above method is that, when the growth rate of silver halide grains is changed according to the scale of the preparation of an emulsion or the ingredients employed, the flow rates of an aqueous silver salt solution and an aqueous halide solution must also be changed to obtain a prescribed amount of fine grains. This leads to the formation of silver halide fine grains differing in size.